The present invention relates to an ionization chamber which can be used as a secondary dosimetric standard and which can be employed as a calibration standard for measuring the photon radiation energy absorbed in body tissue, in water or air.
Secondary standards are measuring instruments which have superior metrological properties and have been calibrated by direct comparison with primary standards, that is to say national calibration standards which have been prepared by primary laboratories and verified by linking them to international primary standards. Such instruments require a high accuracy of measurement, long-term stability and reproducibility.
To obtain the long-term stability demanded for secondary standards, a good long-term volume stability is especially necessary for secondary-standard ionization chambers (SSI). At the same time, to obtain the requisite accuracy of measurement, in addition to the volume stability, a high invariability of the electric fields in the ionization chamber is also necessary since variations of the latter, with the same level of energy dose absorbed in the ionization chamber, cause differences in the ionization currents used as the measured parameter. Moreover, for determining the energy dose by means of ionization chambers, it is necessary to use chamber wall materials which are largely equivalent to air, water or tissue. Likewise, substantial independence of the energy must be demanded, in particular for the determination of the energy dose of mixed radiation, for example, due to scattered radiation and/or rays of different .gamma.-energy. These demands are not met by ionization chambers which are intended for routine use, in particular, in medicine and in radiation protection.
Ionization chambers with chamber walls which are closed on all sides and are used at the same time as outer electrodes, are used as SSI. Such chambers are operated with air under normal ambient conditions. To produce the same atmospheric condition in the ionization chamber as the conditions which can be determined by measurement technology in the surroundings, there is a small "air compensation orifice" in the chamber wall. To obtain the requisite long-term volume stability, SSI have hitherto been manufactured only from graphite with an inner electrode of aluminum, since the demand for the requisite mechanical stability cannot be met with other materials likewise used for ionization chambers, such as, for example, mixtures of polyethylene and plyformaldehyde (French Pat. No. 1,360,381).
Graphite SSI are optimized so that they are suitable for calibration by comparison with the open air chambers used as the primary standard; the ion dose (exposure) being used as the measured parameter.
Since the new fixing of the internationally recognized measurement units of the SI system of units, the Rontgen (R) has been deleted as a unit of measurement. As the new unit, the Gray unit (Gy) has been fixed in which, in contrast to the ion dose measured in Rontgen, the particular absorbed energy dose in measured.
Since the chamber walls of the graphite ionization chambers, do not consist of a material equivalent to air, water or tissue, the calibration of the graphite chambers, previously calibrated by comparison with open air chambers, for the determination of the dose in water or tissue is made much more difficult because of the variable dependence on the energy, caused by the material. The reason for this is that, on the one hand, the effective atomic number of graphite is far away from that of human tissue and, on the other hand, considerable errors can arise in measurements in phantoms. This is because these chambers in the phantom vary considerably their energy dependence, which is optimized for open air measurements, due to the inhomogeneity caused by the combination of a graphite outer wall and an aluminum inner electrode. Aluminum, the atomic number of which is higher than that of air or tissue, is used for compensating the unduly low atomic number of graphite. This compensation is possible in each case only for a small energy range. To reach the secondary electron equilibrium required for the measurement, wall thicknesses of a few mm are necessary for gamma radiation, which wall thicknesses, on the other hand, already lead to intolerably high absorption losses in the measurement of X-radiation due to the low energy of the latter. The graphite SSI hitherto used are optimized for X-radiation and therefore require additional wall-reinforcing caps for the measurement of gamma radiation. This type of measurement, however, causes undesirable inaccuracies in the measurement, in particular in the boundary regions.
A further disadvantage in the construction of the ionization chambers hitherto used is the generation of uncontrollable measurement errors due to electrical phenomena in the boundary layer between the electrodes, which are electrically separated from one another, in the neck part of the chamber. The electric potential of this insulating layer varies as a function of the atmospheric humidity and temperature in the interior of the chamber, whereby the electric field in the interior of the chamber is distorted. This leads to a variation of the ionization current of the inner electrode. These measurement errors which, in general, can be disregarded in normal ionization chambers are unacceptable for secondary standards.